Scandium, which has the smallest atomic number among the rare earth elements, has been used as a material for metal halide lamps, an added element in alloys, an added element in catalyst ceramics, and the like. However, scandium is expensive, the output thereof is limited, and separation and refinement are difficult, and thus the application of scandium has been limited.
However, nickel oxide ore such as laterite ore has been known to contain very small amounts of scandium. The scandium contained in nickel oxide ore can be recovered from the leachate obtained by adding sulfuric acid to the nickel oxide ore and pressurized extracting.
For example, Patent Document 1 shows that it is possible to recover nickel and scandium from oxide ores by performing: (A) a leaching step of leaching the oxide ore with an acid under high temperature and high pressure to obtain a leachate containing nickel and scandium; (B) a first neutralizing step of removing iron and aluminum in the leachate as precipitates by adjusting the pH to the range of 2 to 4 by adding a neutralizing agent to this leachate; (C) a second neutralizing step of recovering scandium in the solution as a precipitate by adjusting the pH to more than 4 to 7.5 by adding a neutralizing agent to the solution after removing the precipitates in the first neutralizing step; and (D) a third neutralizing step of recovering nickel in the solution as a precipitate by adjusting the pH to more than 7.5 by further adding neutralizing agent.
However, various problems arise when trying to industrially operate with the method described in Patent Document 1. For example, since the pH adjustment range in the first neutralizing step and the pH adjustment range in the second neutralizing step are adjacent, there is a possibility of scandium also precipitating along with iron and aluminum in the first neutralizing step, leading to an extraction rate decline for scandium, and there is a possibility of iron and aluminum precipitating along with scandium in the second neutralizing step, leading to a purity decline for scandium, and neither are preferable. In addition, although large amounts of precipitates are generated by adding a neutralizing agent, in general, the characteristics of the precipitates obtained by adding an alkali to an acid are unstable and poor in filterability, and there is a possibility of being accompanied with a cost increase such as the enlargement of the equipment scale. For this reason, it is preferable to decrease the number of times of the neutralizing step as much as possible, and it has been proposed to selectively separate only scandium by a means such as solvent extraction from a solution containing scandium.
Patent Document 2 shows that high purity scandium oxide is obtained by extracting scandium component into organic solvent by adding the organic solvent to an aqueous-phase scandium-containing solution containing, in addition to scandium, at least one among iron, aluminum, calcium, yttrium, manganese, chromium and magnesium, then in order to separate trace components extracted along with scandium in the organic solvent, scrubbing is performed by adding a hydrochloric acid aqueous solution, and after removing the trace components, the scandium remaining in the organic solvent is made into a slurry containing as Sc(OH)3 by adding a sodium hydroxide aqueous solution to the organic solvent, the Sc(OH)3 obtained by filtering this is dissolved with hydrochloric acid to obtain a scandium chloride aqueous solution, and scandium oxalic acid precipitate is formed by adding to this oxalic acid, the precipitate is filtered, and after separating the trace impurities in the filtrate, calcining.
However, in the case of employing the method of Patent Document 2, not only scandium, but also impurity components are extracted in the organic solvent to an extent that cannot be ignored. Calcium, magnesium, etc. are abundantly present particularly in the leachate obtained by acid leaching and neutralizing nickel oxide ore, and for this reason, in addition to the problem of labor and cost required in scrubbing, there is also a further problem in the treatment of drainage generated accompanying scrubbing.
In addition, Patent Document 3 discloses an extraction agent called DODGAA, which has a diglycol amic acid backbone. According to this extraction agent, the solubility in water is extremely small, and as well as complete incineration being possible, comparing with existing phosphorous-based compounds, it has superior rare earth metal extractability and selective separability, as well as the synthesis cost being low.
However, since scandium is very sensitive to pH, upon extracting scandium even with the above-mentioned extraction agent shown in Patent Document 3, a practical extraction rate is not obtained if maintaining the pH at a constant or higher. Moreover, in the pH range suited to the extraction of scandium, the extraction rate of not only scandium, but also calcium and magnesium increase, and thus it is difficult to selectively separate only scandium.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2000-313928
[Patent Document 2] Japanese Unexamined Patent Application, Publication No. H9-291320
[Patent Document 3] Japanese Unexamined Patent Application, Publication No. 2007-327085